We have been investigating the role of postoperative radiotherapy in locally advanced colorectal lesions (B2, C1, C2) to determine if radiation can reduce the incidence of local recurrence. 95 patients with colorectal lesions have received postoperative radiotherapy. Patients with rectosigmoid lesions received 4500 to 5000 rads tumor dose to the pelvis and paraortic lymph nodes by an inverted T-field. Patients with higher lesions received treament to the whole abdomen by the moving strip technique. Only two of 57 patients with rectosigmoid lesions and only five of 38 with colonic lesions have recurred locally. Thus, less than 10% have developed local recurrences. We will continue the present protocol for patients with rectal and low sigmoid lesions. However, for patients with tumors located above the mid-sigmoid (who have a higher percentage of metastases), we are randomizing patients between postoperative irradiation to the whole abdomen and postoperative chemotherapy (5FU and MeCCNU). Large field irradiation did not appear to alter in vivo and in vitro parameters of cell mediated immunity. There is a suggestion that radiotherapy may increase the level of circulating immune complexes. We will continue our investigations on the effect of large field irradiation on cell mediated immunity and the release of immune complexes.